Many types of machines and machine accessories have components which move relative to one another or to some other part of the machine. Components involving linear or pivoting relative movement are abundant.
One type of machine having parts involving relative pivoting movement is known as a dragline, a type of excavating machine equipped with an extending boom from which is suspended a digging bucket. To excavate, the bucket is placed on the ground away from the machine. With its teeth rearward (facing the machine), such bucket is drawn (or "dragged") toward such machine by taut cable. When the bucket is filled, it is hoisted by other taut cables and the machine is then rotated to dump the bucket contents on a spoil pile.
During digging and later bucket emptying, such bucket must assume a variety of "attitudes," i.e., a horizontal position to dig and a steeply-angled or vertical position to empty. The hoisting cables are attached to chain-link "rigging" which, in turn, is attached to an eye-like link at the end of the rigging. The link couples to a clevis or (in another configuration) to a bracket-like bucket trunnion using a pin which engages the clevis and link or the bucket trunnion and link, as the case may be. Such arrangement permits the bucket to be supported by the rigging and the hoisting cables and yet assume the positions required for digging and bucket emptying.
The trunnion link, bucket and bucket trunnion are subjected to extraordinary wear and impact. (In fact, it is difficult to envision the rigors of bucket service without actually having seen a dragline in operation.)
Notwithstanding the rigors of hard service, it is important that the pivot pin be securely retained so that the trunnion and link do not separate from one another. But when the pin is worn to the point that it must be replaced, the structure retaining such pin is preferably configured to be quick and easy to remove and replace. A large dragline may represent a capital expenditure of several million dollars--machine downtime is very expensive.
While generally satisfactory, prior art arrangements for retaining a pin have some deficiencies, at least with respect to ease of parts replacement including pin or link replacement. In one arrangement, the cylindrical pivot pin has a cap-like head on either end and one of such heads is welded to the pin after the bucket and rigging are assembled in the field. The heads prevent the pin from falling out.
There are two disadvantages to this approach. One is that to replace a pin, one has to "cut" the welded head from the pin (by using an acetylene torch, for example) and weld a head on an end of the replacement pin after such pin is installed. This is time consuming and requires that flame cutting equipment and a person skilled as a welder be available on the site.
Another disadvantage to the "welded head" approach is that the hardness of the pin cannot exceed some maximum, e.g., about 400-450 Brinell. A pin of such hardness does not wear as well as desired and it must be replaced with a frequency that with the advent of the invention is unnecessary.
In an arrangement involving a link, clevis and retaining pin, another approach to pin retention involves using a cylindrical pin without welded heads and mounting a cover at each side of the clevis and over each end of the pin. The cover mounts tongue-and-groove fashion on straight tracks affixed to the clevis. After being properly positioned, the covers are welded in place.
While this approach avoids pin welding (and therefore permits using a pin having a hardness of about 650 Brinell), it does not avoid welding altogether. As a consequence, the user of such approach is likely to experience at least some of the downtime, equipment and personnel availability problems mentioned above.
Another prior art arrangement of the type involving a bucket-mounted trunnion, a link and a pin securing the trunnion and link to one another has a similar disadvantage. In this arrangement, the pin is prevented from working its way out in one direction by a pin shoulder which bears against the bucket. The pin is prevented from working its way out in the other direction by a welded-in-place cover plate of the type described above.
Such arrangement is attended by the disadvantages mentioned above with respect to the welded-cover approaches. And it has been found that when a bucket-mounted trunnion is used to connect a link and bucket to one another, the outer surface of the trunnion becomes caked with mud which hardens (over the welded cover) and is very difficult to remove.
An improved pin-retention device and method which avoid welding when installing or replacing a pin in the field, which reduce downtime and which involve only commonly-available tools and personnel skills would be an important advance in the art.